Macrophages possess cytostatic and cytotoxic activities against neoplastic cells. During growth of syngeneic tumors in rats or mice, however, there occurs a progressive inhibition of macrophage accumulation in induced peritoneal exudates, in subcutaneous tissues, and in the tumor itself. The proposed work seeks to ascertain the factors involved in this abnormal monocyte inflammatory reaction. Further, we will determine if the abnormality exists in spontaneously arising tumors as well as those transplanted and whether the abnormality is induced only by neoplastic growth.